策略模式（策略不同类，场景不同策略不同）

public interface Strategy {  
     public void operate();  
} 

public class BackDoor implements IStrategy {  
     @Override  
     public void operate() {  
          System.out.println("找乔国老帮忙，让吴国太给孙权施加压力，使孙权不能杀刘备");  
     }  
}

1 public class GivenGreenLight implements IStrategy {  
2      @Override  
3      public void operate() {  
4           System.out.println("求吴国太开个绿灯，放行");  
5      }  
6 } 

1 public class BlackEnemy implements IStrategy {  
2      @Override  
3      public void operate() {  
4           System.out.println("孙夫人断后，挡住追兵");  
5      }  
6 }

 1 public class Context {  
 2      private Strategy strategy;  
 3      //构造函数，要你使用哪个妙计  
 4      public Context(Strategy strategy){  
 5           this.strategy = strategy;  
 6      }  
 7      public void setStrategy(Strategy strategy){  
 8           this.strategy = strategy;  
 9      }  
10      public void operate(){  
11           this.strategy.operate();  
12      }  
13 }

 1 public class LiuBei {  
 2   
 3 public static void main(String[] args) {  
 4      Context context;  
 5   
 6      System.out.println("----------刚到吴国使用第一个锦囊---------------");  
 7      context = new Context(new BackDoor());  
 8      context.operate();  
 9      System.out.println("\n");  
10   
11      System.out.println("----------刘备乐不思蜀使用第二个锦囊---------------");  
12      context.setStrategy(new GivenGreenLight());  
13      context.operate();  
14      System.out.println("\n");  
15   
16      System.out.println("----------孙权的追兵来了，使用第三个锦囊---------------");  
17      context.setStrategy(new BlackEnemy());  
18      context.operate();  
19      System.out.println("\n");  
20      }  
21 }